Method, base station, and user terminal for using location information of user terminal

ABSTRACT

A base station allocates radio resources used for device-to-device D2D communication based on location information indicating a geographical location of a user terminal located in a cell of a base station. The base station notifies the allocated radio resources to the user terminal by a unicast manner, and the user terminal uses the allocated radio resources to transmit a signal for the D2D communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.14/903,794 filed Jan. 8, 2016, which is the U.S. National Phase ofInternational Application No. PCT/JP2014/068136 filed Jul. 8, 2014,which claims the benefit of Japanese Patent Application No. 2013-144025filed Jul. 9, 2013, the entire contents of which are incorporated hereinby reference.

TECHNICAL FIELD

The present disclosure relates to a network apparatus and acommunication control method which are used in a mobile communicationsystem that supports device-to-device (D2D) communication.

BACKGROUND ART

In the 3rd Generation Partnership Project (3GPP) that is a mobilecommunication system standardization project, introduction ofdevice-to-device (D2D) communication into Release 12 as a new functionis under consideration (see Non Patent Literature 1).

In the D2D communication, a plurality of nearby user terminals performdirect inter-terminal communication without passing through a network.On the other hand, in cellular communication that is normalcommunication of a mobile communication system, user terminals performcommunication via a network.

The user terminal transmits and receives a discovery signal used fordiscovering a nearby user terminal in order to perform the D2Dcommunication. After the discovery process, the user terminal performsthe D2D communication with the nearby user terminal.

CITATION LIST Non Patent Literature

Non Patent Literature 1: 3GPP Technical Report “TR 22.803 V12.1.0,”March, 2013

SUMMARY

In the mobile communication system that supports the D2D communication,it is necessary to secure radio resources (hereinafter, “discoveryresources”) used for transmission and reception of the discovery signal,separately from radio resources (hereinafter, “D2D communicationresources”) used for transmission and reception of user data in the D2Dcommunication.

However, when the discovery resources are secured, it is possible toincrease a probability that the nearby user terminal will be discoveredsuccessfully, but there is a problem in that since the D2D communicationresources or cellular communication resources are relatively reduced, asystem throughput decreases.

In this regard, it is an object of the present disclosure to provide anetwork apparatus and a communication control method, which are capableof appropriately setting the discovery resources.

A method for a mobile communication system according to the presentdisclosure supports a device-to-device (D2D) communication that isdirect inter-terminal communication. The method comprises transmittinglocation information indicating a geographical location of a userterminal located in a cell of a base station, from the user terminal tothe base station, allocating, at the base station, radio resources tothe user terminal based on the location information, where the radioresources are used by the user terminal to transmit a signal for the D2Dcommunication, notifying the allocated radio resources to the userterminal by a unicast manner, receiving, at the user terminal, a unicastsignal indicating the radio resources allocated by using the locationinformation, from the base station; and using the allocated radioresources to transmit the signal for the D2D communication.

A base station for a mobile communication system according to thepresent disclosure supports a device-to-device (D2D) communication thatis direct inter-terminal communication. The base station comprises acontroller configured to acquire location information indicatinggeographical location of a user terminal located in a cell of the basestation, from the user terminal, allocate radio resources to the userterminal based on the location information, where the radio resourcesare used by the user terminal to transmit a signal for the D2Dcommunication, and notify the allocated radio resources to the userterminal by a unicast manner.

A user terminal for a mobile communication system according to thepresent disclosure supports a device-to-device (D2D) communication thatis direct inter-terminal communication. The user terminal comprises acontroller configured to transmit location information indicating ageographical location of the user terminal located in a cell of a basestation, to the base station, receive a unicast signal indicating radioresources allocated by using the location information, from the basestation, the unicast signal being specific to the user terminal, and usethe allocated radio resources to transmit a signal for the D2Dcommunication.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram illustrating an LTE system accordingto an embodiment.

FIG. 2 is a block diagram illustrating a UE according to an embodiment.

FIG. 3 is a block diagram illustrating an eNB according to anembodiment.

FIG. 4 is a protocol stack diagram illustrating a radio interfaceaccording to an embodiment.

FIG. 5 is a configuration diagram illustrating a radio frame accordingto an embodiment.

FIG. 6 is a diagram illustrating D2D communication according to anembodiment.

FIG. 7 is a diagram illustrating a format of discovery resourcesaccording to an embodiment.

FIGS. 8A and 8B are diagrams illustrating a first operation patternaccording to an embodiment.

FIG. 9 is a sequence diagram of the first operation pattern according toan embodiment.

FIG. 10 is a flowchart illustrating a flow of calculating a discoverytime in the first operation pattern according to an embodiment.

FIGS. 11A and 11B are diagrams illustrating a second operation patternaccording to an embodiment.

FIG. 12 is a sequence diagram illustrating a third operation patternaccording to an embodiment.

FIG. 13 is a flowchart illustrating a flow of calculating a discoverytime in the third operation pattern according to an embodiment.

FIGS. 14A and 14B are diagrams illustrating a fourth operation patternaccording to an embodiment.

FIG. 15 is a diagram illustrating an interference between UEs thatdiffer in a setting of a discovery time.

FIG. 16 is a diagram illustrating another format of discovery resources.

FIG. 17 is another flowchart illustrating a flow of calculating adiscovery time in the third operation pattern according to anembodiment.

DESCRIPTION OF EMBODIMENTS Overview of Embodiments

A network apparatus according to embodiments is included in a network ofa mobile communication system that supports device-to-device (D2D)communication that is direct inter-terminal communication. The networkapparatus includes a controller configured to set discovery resourcesthat are radio resources used for transmission or reception of adiscovery signal for performing the D2D communication. The controllercontrols a quantity of the discovery resources based on information on auser terminal existing in a target area that is a setting target area ofthe discovery resources.

In the embodiments, the target area is a cell of the mobilecommunication system. The controller controls the quantity of thediscovery resources set to the cell based on information on the userterminal existing in the cell.

In the embodiments, the information on the user terminal is informationindicating number of user terminals existing in the target area.

In the embodiments, the information on the user terminal is informationindicating a density of user terminals existing in the target area.

In the embodiments, the information on the user terminal is informationindicating an attribute of the user terminal existing in the targetarea.

In the embodiments, the information on the user terminal is informationindicating transmission power of the discovery signal in the userterminal existing in the target area.

In the embodiments, the information on the user terminal is informationon a result of a discovery process using the discovery signal in theuser terminal existing in the target area.

In the embodiments, the information on the user terminal is informationindicating a size of the cell in which the user terminal exists.

A communication control method according to embodiments is used in amobile communication system that supports device-to-device (D2D)communication that is direct inter-terminal communication. Thecommunication control method includes a step of setting, by a networkapparatus included in a network of the mobile communication system,discovery resources that are radio resources used for transmission orreception of a discovery signal for performing the D2D communication. Inthe step of the setting, the network apparatus controls a quantity ofthe discovery resources based on information on a user terminal existingin a target area that is a setting target area of the discoveryresources.

Embodiment

Hereinafter, an embodiment in which the present disclosure is applied toan LTE system will be described.

(System Configuration)

FIG. 1 is a configuration diagram of an LTE system according to a firstembodiment. As illustrated in FIG. 1, the LTE system includes aplurality of UEs (User Equipments) 100, E-UTRAN (Evolved-UMTSTerrestrial Radio Access Network) 10, and EPC (Evolved Packet Core) 20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobilecommunication device and performs radio communication with a cell (aserving cell) with which a connection is established. Configuration ofthe UE 100 will be described later.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes a plurality of eNBs (evolved Node-Bs) 200. The eNB 200corresponds to a base station. The eNBs200 are connected mutually via anX2 interface. Configuration of the eNB200 will be described later.

The eNB 200 manages one or a plurality of cells and performs radiocommunication with the UE 100 which establishes a connection with thecell of the eNB 200. The eNB 200 has a radio resource management (RRM)function, a routing function for user data, and a measurement controlfunction for mobility control and scheduling, and the like. It is notedthat the “cell” is used as a term indicating a minimum unit of a radiocommunication area, and is also used as a term indicating a function ofperforming radio communication with the UE 100.

The EPC 20 corresponds to a core network. The EPC 20 includes aplurality of MME (Mobility Management Entity)/S-GWs (Serving-Gateways)300. The MME performs various mobility controls and the like for the UE100. The S-GW performs control to transfer user. MME/S-GW 300 isconnected to eNB 200 via an S1 interface.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes plural antennas 101, a radio transceiver 110, a userinterface 120, a GNSS (Global Navigation Satellite System) receiver 130,a battery 140, a memory 150, and a processor 160. The memory 150 and theprocessor 160 constitute a controller. The UE 100 may not have the GNSSreceiver 130. Furthermore, the memory 150 may be integrally formed withthe processor 160, and this set (that is, a chip set) may be called aprocessor 160′.

The plural antennas 101 and the radio transceiver 110 are used totransmit and receive a radio signal. The radio transceiver 110 convertsa baseband signal (a transmission signal) output from the processor 160into the radio signal and transmits the radio signal from the antenna101. Furthermore, the radio transceiver 110 converts a radio signalreceived by the antenna 101 into a baseband signal (a received signal),and outputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, variousbuttons and the like. The user interface 120 accepts an operation from auser and outputs a signal indicating the content of the operation to theprocessor 160. The GNSS receiver 130 receives a GNSS signal in order toobtain location information indicating a geographical location of the UE100, and outputs the received signal to the processor 160. The battery140 accumulates power to be supplied to each block of the UE 100.

The memory 150 stores a program to be executed by the processor 160 andinformation to be used for a process by the processor 160. The processor160 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signal,and CPU (Central Processing Unit) that performs various processes byexecuting the program stored in the memory 150. The processor 160 mayfurther include a codec that performs encoding and decoding on sound andvideo signals. The processor 160 executes various processes and variouscommunication protocols described later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes plural antennas 201, a radio transceiver 210, a networkinterface 220, a memory 230, and a processor 240. The memory 230 and theprocessor 240 constitute a controller.

The plural antennas 201 and the radio transceiver 210 are used totransmit and receive a radio signal. The radio transceiver 210 convertsa baseband signal (a transmission signal) output from the processor 240into the radio signal and transmits the radio signal from the antenna201. Furthermore, the radio transceiver 210 converts a radio signalreceived by the antenna 201 into a baseband signal (a received signal),and outputs the baseband signal to the processor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication over theX2 interface and communication over the S1 interface.

The memory 230 stores a program to be executed by the processor 240 andinformation to be used for a process by the processor 240. The processor240 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signaland CPU that performs various processes by executing the program storedin the memory 230. The processor 240 executes various processes andvarious communication protocols described later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a layer 1 to a layer 3 of an OSI reference model,wherein the layer 1 is a physical (PHY) layer. The layer 2 includes aMAC (Medium Access Control) layer, an RLC (Radio Link Control) layer,and a PDCP (Packet Data Convergence Protocol) layer. The layer 3includes an RRC (Radio Resource Control) layer.

The PHY layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the PHY layer of the UE 100 and the PHY layer of theeNB 200, use data and control signal are transmitted via the physicalchannel.

The MAC layer performs priority control of data, a retransmissionprocess by hybrid ARQ (HARQ), and the like. Between the MAC layer of theUE 100 and the MAC layer of the eNB 200, user data and control signalare transmitted via a transport channel. The MAC layer of the eNB 200includes a scheduler that determines a transport format of an uplink anda downlink (a transport block size and a modulation and coding scheme)and a resource block to be assigned to the UE 100.

The RLC layer transmits data to an RLC layer of a reception side byusing the functions of the MAC layer and the PHY layer. Between the RLClayer of the UE 100 and the RLC layer of the eNB 200, user data andcontrol signal are transmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane dealing with controlsignal. Between the RRC layer of the UE 100 and the RRC layer of the eNB200, control message (RRC messages) for various types of configurationare transmitted. The RRC layer controls the logical channel, thetransport channel, and the physical channel in response toestablishment, re-establishment, and release of a radio bearer. Whenthere is an RRC connection between the RRC of the UE 100 and the RRC ofthe eNB 200, the UE 100 is in a connected state (an RRC connectedstate), otherwise the UE 100 is in an idle state (an RRC idle state).

A NAS (Non-Access Stratum) layer positioned above the RRC layer performsa session management, a mobility management and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency DivisionMultiplexing Access) is applied to a downlink, and SC-FDMA (SingleCarrier Frequency Division Multiple Access) is applied to an uplink,respectively.

As illustrated in FIG. 5, the radio frame is configured by 10 subframesarranged in a time direction, wherein each subframe is configured by twoslots arranged in the time direction. Each subframe has a length of 1 msand each slot has a length of 0.5 ms. Each subframe includes a pluralityof resource blocks (RBs) in a frequency direction, and a plurality ofsymbols in the time direction. The resource block includes a pluralityof subcarriers in the frequency direction. One subcarrier and one symbolconstitute one resource element.

Among radio resources assigned to the UE 100, a frequency resource canbe specified by a resource block and a time resource can be specified bya subframe (or slot).

In the downlink (DL), an interval of several symbols at the head of eachsubframe is a control region used as a physical downlink control channel(PDCCH) for mainly transmitting a control signal. Furthermore, the otherinterval of each subframe is a region available as a physical downlinkshared channel (PDSCH) for mainly transmitting user data.

In the uplink (UL), both ends in the frequency direction of eachsubframe are control regions used as a physical uplink control channel(PUCCH) for mainly transmitting a control signal. Furthermore, the otherportion of each subframe is a region available as a physical uplinkshared channel (PUSCH) for mainly transmitting user data.

(D2D Communication)

The LTE system according to an embodiment supports the D2D communicationthat is direct inter-terminal communication (inter-UE communication).Here, the description will proceed with a comparison between the D2Dcommunication and cellular communication that is normal communication ofthe LTE system. The cellular communication is a communication mode inwhich a data path passes through a network (the E-UTRAN 10 and the EPC20). The data path refers to a communication path of user data. On theother hand, the D2D communication is a communication mode in which adata path set between the UEs does not pass through a network.

FIG. 6 is a diagram illustrating the D2D communication. In the D2Dcommunication, the data path does not pass through the eNB 200 asillustrated in FIG. 6. The UE 100-1 and the UE 100-2 that are close toeach other directly perform radio communication at low transmissionpower in the cell of the eNB 200. As described above, as the UE 100-1and the UE 100-2 that are close to each other directly perform radiocommunication at low transmission power, it is possible to reduce thepower consumption of the UE 100 to be smaller than in the cellularcommunication and reduce interference to a neighboring cell.

Operation According to Embodiment

Next, an operation according to an embodiment will be described.

(1) Overview of Operation

The UE 100 transmits or receives the discovery signal used fordiscovering the nearby UE 100 in order to perform the D2D communication.After the discovery process, the UE 100 performs the D2D communicationwith the nearby UE 100.

Thus, in the mobile communication system that supports the D2Dcommunication, it is necessary to secure radio resources (the discoveryresources) used for transmission and reception of the discovery signal,separately from radio resources (the D2D communication resources) usedfor transmission and reception of at least user data in the D2Dcommunication. However, when the discovery resources are secured, it ispossible to increase a probability that the nearby UE 100 will bediscovered successfully, but since the D2D communication resources orthe cellular communication resources are relatively reduced, the systemthroughput decreases.

In an embodiment, the eNB 200 sets the discovery resources serving asthe radio resources used for transmission and reception of the discoverysignal in order to perform the D2D communication. For example, the eNB200 sets the D2D communication resources and the discovery resources inthe radio resources secured for the D2D communication in a time divisionmanner. Alternatively, the eNB 200 may set the D2D communicationresources and the discovery resources in a frequency division manner.The following description will proceed with an example in which the D2Dcommunication resources and the discovery resources are set in the timedivision manner.

The eNB 200 controls a discovery resource quantity (duration) based oninformation on the UE 100 existing in a target area serving as adiscovery resource setting target area. For example, the target area isa cell. However, the target area may be a tracking area. Alternatively,instead of controlling the discovery resource quantity for each area,the discovery resource quantity may be controlled for each UE 100.Further, “exist” does not depend on whether the UE 100 is in an idlestate (an RRC idle state) or a connected state (an RRC connected state).

As the eNB 200 controls the discovery resource quantity based on theinformation on the UE 100 existing in the target area as describedabove, it is possible to adaptively set the discovery resource quantityaccording to a state of the UE 100 existing in the target area. Thus,the discovery resources can appropriately be set.

An operation of controlling the discovery resource quantity includesfirst to seventh operation patterns described below. The first toseventh operation patterns will be described in detail later, but anoverview of each operation pattern is described here. The first toseventh operation patterns need not be necessarily independentlyperformed and two or more patterns are combined and performed.

In the first operation pattern, the eNB 200 controls the discoveryresource quantity based on information indicating the number of UEs 100existing in the target area. In the second operation pattern, the eNB200 controls the discovery resource quantity based on informationindicating transmission power of the discovery signal in the UE 100existing in the target area. In the third operation pattern, the eNB 200controls the discovery resource quantity based on information indicatingthe density of the UEs 100 existing in the target area. In the fourthoperation pattern, the eNB 200 controls the discovery resource quantitybased on information on a result of the discovery process using thediscovery signal in the UE 100 existing in the target area. In the fifthoperation pattern, the eNB 200 controls the discovery resource quantitybased on information indicating the size of the cell in which the UE 100exists. In the sixth operation pattern, the eNB 200 controls thediscovery resource quantity based on the information indicating theattribute of the UE 100 existing in the target area. In the seventhoperation pattern, the eNB 200 controls the discovery resource quantitybased on the information indicating the frequency band used fortransmission and reception of the discovery signal by the UE 100existing in the target area.

(2) Format of Discovery Resources

FIG. 7 is a diagram illustrating a format of the discovery resourcesaccording to an embodiment.

As illustrated in FIG. 7, in a subframe secured for the D2Dcommunication, the D2D communication resources and the discoveryresources are set in the time division manner. The D2D communicationresources are the radio resources used for transmission and reception ofuser data (and a control signal) in the D2D communication. The discoveryresources are the radio resources used for transmission and reception ofthe discovery signal. For example, code division multiplexing using anorthogonal code is applied to the discovery resources.

In an example of FIG. 7, an interval of several symbols from the headcorresponds to the discovery resources, and the remaining intervalcorresponds to the D2D communication resources. Hereinafter, theinterval corresponding to the discovery resources is referred to as a“discovery time,” and the interval corresponding to the D2Dcommunication resources is referred to as a “D2D communication time(communication time).” A duration obtained by adding the discovery timeand the D2D communication time is a predetermined duration (a subframelength in the example of FIG. 7). Thus, when the discovery time isincreased, the D2D communication time is relatively decreased, and theD2D communication capacity (the throughput of the D2D communication)decreases. On the other hand, when the discovery time is decreased, theD2D communication time is relatively increased, the D2D communicationcapacity (the throughput of the D2D communication) is improved.

In the example of FIG. 7, the discovery resources and the D2Dcommunication resources can be set in units of symbols within thesubframe in the time division manner, but the present disclosure is notlimited thereto, and the discovery resources and the D2D communicationresources may be set in units of subframes within the radio frame in thetime division manner.

In order to transmit and receive the discovery signal, the UE 100 areallocated the discovery resources (time and frequency resources) and anorthogonal code from the eNB 200. The UE 100 transmits and receives thediscovery signal using the discovery resources and the orthogonal codeallocated from the eNB 200.

(3) First Operation Pattern

In the first operation pattern, the eNB 200 controls the discovery timebased on the information indicating the number of UEs 100 existing inthe target area.

For example, the information indicating the number of UEs 100 includethe number of UEs 100 in the connected state in its own cell or thenumber of UEs 100 in the idle state in the tracking area including itsown cell. The eNB 200 can acquire the number of UEs 100 in the idlestate from the MME 300. The number of UEs 100 may be limited to thenumber of UEs 100 that support the D2D communication. In this case, theeNB 200 acquires information indicating whether or not the D2Dcommunication is supported from the UE 100.

FIGS. 8A and 8B are diagrams illustrating the first operation pattern.As illustrated in FIGS. 8A and 8B, when the number of UEs 100 is large,the eNB 200 increases the discovery time and decreases the discoverytime when the number of UEs 100 is small. By controlling the discoverytime according to the number of UEs 100, it is possible to set thediscovery resource quantity suitable for the number of UEs 100.

Specifically, when the code length of the orthogonal code applied in thediscovery time is variable, both the discovery time and the code lengthare increased. By increasing the code length, it is possible to increasethe number of available orthogonal codes, and thus it is possible toallocate the orthogonal code for the discovery signal to the more UEs100.

On the other hand, when the code length of the orthogonal code appliedin the discovery time is fixed, the number of available orthogonal codesis fixed, and thus the number of UEs accommodatable in the discoverytime is increased by increasing the discovery time. By increasing thediscovery time to be n times a duration (a unit duration) correspondingto one orthogonal code, it is possible to increase the number of UEsaccommodatable in the discovery time by n times.

Further, when the number of UEs 100 is small, both the discovery timemay be decreased (the D2D communication time may be increased), and thefrequency resources in the discovery time may be decreased as well. Forexample, when the number of UEs 100 is small, some resource blocksrather than all resource blocks in the subframe secured for the D2Dcommunication are used as the discovery resources and the D2Dcommunication resources.

FIG. 9 is a sequence diagram illustrating the first operation pattern.As illustrated in FIG. 9, in step S11, the eNB 200 calculates thediscovery time based on the number of UEs 100 in its own cell. Adiscovery time calculation flow will be described later. In step S12,the eNB 200 notifies the UE 100 in its own cell of the calculateddiscovery time in a unicast or broadcast manner. The UE 100 may transmita response to the notified discovery time to the eNB 200 (step S13).

FIG. 10 is a flowchart illustrating the discovery time calculation flowin the first operation pattern. As illustrated in FIG. 10, in step S111,the eNB 200 determines whether or not the UE 100 exists in its own cell.When a determination result in step S111 is “NO,” in step S112, the eNB200 sets the discovery time to zero. When a determination result in stepS111 is “Yes,” in step S113, the eNB 200 determines whether or not thenumber of UEs 100 in its own cell is equal to or more than a maximum ofthe number of UEs accommodatable in the discovery time. When adetermination result in step S113 is “YES,” in step S114, the eNB 200sets the discovery time to a maximum value. When a determination resultin step S113 is “NO,” in step S115, the eNB 200 determines whether ornot the number of UEs 100 in its own cell exceeds a maximum of thenumber of UEs to which code division multiplexing can be applied. When adetermination result in step S115 is “YES,” in step S117, the eNB 200sets a value (a decimal is rounded up) obtained by dividing the numberof UEs 100 in its own cell by a maximum of the number of UEsaccommodatable in a minimum discovery time as the discovery time. When adetermination result in step S115 is “NO,” in step S116, the eNB 200sets a value that is twice the minimum discovery time as the discoverytime.

(4) Second Operation Pattern

In the second operation pattern, the eNB 200 controls the discoveryresource quantity based on the information indicating the transmissionpower of the discovery signal in the UE 100 existing in the target area.

When the transmission power of the discovery signal is managed by theeNB 200, the eNB 200 can use information of the managed transmissionpower of the discovery signal. When the transmission power of thediscovery signal is decided by the UE 100, the eNB 200 acquire theinformation indicating the transmission power of the discovery signalfrom the UE 100 and use the acquired information. The informationindicating the transmission power of the discovery signal may be astatistic (an average value, a maximum value, a minimum value, a modevalue, or the like) of the transmission power of the discovery signal inthe UE 100 in its own cell.

FIGS. 11A and 11B are diagrams illustrating the second operationpattern. As illustrated in FIGS. 11A and 11B, the eNB 200 increases thediscovery time when the transmission power of the discovery signal islow but decreases the discovery time when the transmission power of thediscovery signal is high. As the transmission power of the discoverysignal decreases, an arrival range of the discovery signal decreases,and thus a probability that the discovery process will be performedsuccessfully decreases. On the other hand, as the discovery timeincreases, the probability that the discovery process will be performedsuccessfully increases. Thus, when the transmission power of thediscovery signal is low, the probability that the discovery process willbe performed successfully can be maintained at a certain level byincreasing the discovery time.

A sequence of notifying the UE 100 of the discovery time decided by theeNB 200 is similar to that in the first operation pattern.

(5) Third Operation Pattern

An example of the third operation pattern will now be described withreference to FIGS. 12, 13 and 17. In the third operation pattern, theeNB 200 controls the discovery resource quantity based on theinformation indicating the density of the UEs 100 existing in the targetarea. For example, the information indicating the density of the UEs 100is an inter-UE path loss or an inter-UE distance based on UE positioninformation. For instance, in step S301 in FIG. 17, each UE 100transmits a reference signal of known transmission power, and thus adifference between reception power and transmission power when thereference signal is received in each UE 100 may be acquired from each UE100 as the inter-UE path loss. Further, the eNB 200 may use GNSSposition information acquired from the UE 100 as the UE positioninformation.

The eNB 200 increases the discovery time when the density of the UEs 100in its own cell is low and decreases the discovery time when the densityof the UEs 100 in its own cell is high. As the density of the UEs 100decreases, the probability that the discovery process will be performedsuccessfully decreases. On the other hand, as the discovery timeincreases, the probability that the discovery process will be performedsuccessfully increases. Thus, when the density of the UEs 100 is low,the probability that the discovery process will be performedsuccessfully can be maintained at a certain level by increasing thediscovery time. Alternatively, in order to discover all UEs within acertain range, the discovery time may be increased when the density ofthe UEs 100 in its own cell is high, and the discovery time may bedecreased when the density of the UEs 100 in its own cell is low.

FIG. 12 is a sequence diagram illustrating the third operation pattern.Here, an example in which the discovery time is set for each UE 100based on the inter-UE path loss will be described. As illustrated inFIG. 12, in step S21, the eNB 200 requests the UE 100 in its own cell(the UEs 100-1 to 100-3) to transmit the inter-UE path loss. In stepS22, each of the UEs 100-1 to 100-3 transmits a list of inter-UE pathlosses to the eNB 200. In step S23 (also step S302 in FIG. 17), the eNB200 decides the discovery time based on the list of the inter-UE pathlosses for each of the UEs 100-1 to 100-3. A discovery time decisionflow will be described later. In step S24 (also step S303 in FIG. 17),the eNB 200 notifies the UEs 100-1 to 100-3 of the decided discoverytime in the unicast manner, which is thus received by the UEs 100-1 to100-3 (step S304 in FIG. 17). The UEs 100-1 to 100-3 may transmit aresponse to the notified discovery time to the eNB 200, and can thus usethe discovery resource (step S305 in FIG. 17).

FIG. 13 is a flowchart illustrating the discovery time calculation flowin the third operation pattern. As illustrated in FIG. 13, a process ofsteps S231 to S234 is performed by each of the UEs 100. In step S231,the eNB 200 determines whether or not the number of UEs that can bediscovered in the minimum discovery time is equal to or more than aminimum of the number of discoverable UEs based on the list of theinter-UE path losses acquired from the target UE 100. For example, whena value (that is, estimated reception power of the discovery signal)obtained by subtracting the inter-UE path loss from the transmissionpower of the discovery signal is equal to or more than a thresholdvalue, it can be determined that it is possible to discover theneighboring UE 100 corresponding to the inter-UE path loss. When adetermination result in step S231 is “NO,” a loop for deciding thediscovery time starts, and when the discovery time is less than themaximum discovery time (NO in step S232), the unit discovery time isadded to the discovery time (step S233). When the number of UEs that canbe discovered in the discovery time is equal to or more than a minimumof the number of discoverable UEs, the process gets out of the loop, andthe discovery time is stored (step S234).

(6) Fourth Operation Pattern

In the fourth operation pattern, the eNB 200 controls the discoveryresource quantity based on the information on the result of thediscovery process using the discovery signal in the UE 100 existing inthe target area. For example, a result of the discovery process isreported from the UE 100 to the eNB 200, and thus the eNB 200 can useinformation on the result of the discovery process.

FIGS. 14A and 14B are diagrams illustrating the fourth operationpattern. As illustrated in FIGS. 14A and 14B, the eNB 200 increases thecurrent discovery time when the UE 100 has discovered no nearby UE in aprevious discovery process, and decreases the current discovery timewhen the UE 100 has discovered a nearby UE in the previous discoveryprocess. Alternatively, a target value of the number of nearby UEsdiscovered in the discovery process may be set, and the discovery timemay be adjusted to become the target value. For example, a method ofincreasing the discovery time until it reaches a minimum of the numberof discoverable UEs and decreasing the discovery time until it fallsbelow a maximum of the number of discoverable UEs may be employed.

(7) Fifth Operation Pattern

In the fifth operation pattern, the eNB 200 controls the discoveryresource quantity based on the information indicating the size of thecell in which the UE 100 exists. The information indicating the size ofthe cell may be a cell type (a macro cell, a pico cell, a femto cell) ormay be information indicating a radius, a diameter, or transmissionpower of the cell.

For example, as the size of its own cell increases, the eNB 200increases the discovery time so that the arrival range of the discoverytime is increased. As the size of its own cell decreases, the eNB 200decreases the discovery time so that the arrival range of the discoverytime is decreased.

(8) Sixth Operation Pattern

In the sixth operation pattern, the eNB 200 controls the discoveryresource quantity based on the information indicating the attribute ofthe UE 100 existing in the target area.

The attribute of the UE 100 refers to a contract condition (for example,a contract in which “the UE can be discovered at a distance of up to 10m or 20 m” or “up to 10 UEs or 20 UEs can be discovered.”).Alternatively, the attribute of the UE 100 may be a type of the UE 100(for example, a public safety UE or a common UE). The eNB 200 mayacquire the information indicating the attribute of the UE 100 from theUE 100 and use the acquired information.

For example, for the UE 100 having a contract condition related to thediscovery process, the eNB 200 adjusts the discovery time so that thecontract condition is satisfied. Further, for the public safety UE, theeNB 200 increases the discovery time to be higher in a success rate ofthe discovery process than for the common UE. Specifically, a requiredarrival range of the discovery signal differs according to a type of UE.Further, when the discovery signals to which the orthogonal codes of thedifferent code lengths are applied are used together in the same cellwithout dividing resources, interference occurs. In order to avoid it,the code length of the orthogonal code applied to the discovery signalis set to the same length, the discovery time is increased, and thenumber of repetitions in repetitive transmission of the discovery signalis changed according to a type of UE. Further, a reception duration ofthe discovery time is decided according to a type of UE. Accordingly,the discovery range required for each UE can be implemented.

(9) Seventh Operation Pattern

In the seventh operation pattern, the eNB 200 controls the discoveryresource quantity based on the information indicating the frequency bandused for transmission and reception of the discovery signal by the UE100 existing in the target area.

When the frequency band used for transmission and reception of thediscovery signal is specified in the same way, the eNB 200 can useinformation indicating the specified frequency bands. When the frequencyband used for transmission and reception of the discovery signal are setfor each UE 100, the UE 100 reports the frequency band used fortransmission and reception of the discovery signal to the eNB 200, andthe eNB 200 can use the information indicating the frequency band usedfor transmission and reception of the discovery signal by the UE 100.

Generally, as the frequency band is low, a radio wave is successfullypropagated, and thus the eNB 200 decreases the discovery time when thefrequency band used for transmission and reception of the discoverysignal is low. On the other hand, the eNB 200 increases the discoverytime when the frequency band used for transmission and reception of thediscovery signal is high.

Conclusion of Embodiment

As described above, the eNB 200 controls the discovery time based on theinformation on the UE 100 existing in the target area. As a result, thediscovery time can adaptively be set according to the state of the UE100 existing in the target area. Thus, the discovery time canappropriately be set.

Other Embodiments

In the above embodiment, the eNB 200 has been described as an example ofthe network apparatus according to the present disclosure, but thenetwork apparatus according to the present disclosure is not limited tothe eNB 200 and may be a higher-level device (the MME 300, the OAM, orthe like) of the eNB 200.

In the above embodiment, the discovery time has been described as beingset in units of tracking areas, units of cells, or units of UEs, butwhen a plurality of UEs 100 that differ in a setting of the discoverytime get closer to each other, an interference problem is likely tooccur. FIG. 15 is a diagram illustrating interference between UEs thatdiffer in a setting of the discovery time. As illustrated in FIG. 15,the discovery time for the UE to which the long discovery time is setoverlaps (collides with) a portion of the D2D communication time inanother UE to which the short discovery time is set. At a receptionside, it is difficult to decode the overlapping portion. Thus, the eNB200 (or the UE 100) that has detected interference caused by theoverlapping may employ any one of the following interference avoidancemethods. A first method is a method of changing transmission power ofUEs that interfere with each other. In this case, a priority may begiven to the discovery signal (the discovery time), or a priority may begiven to the user data (the D2D communication time). A second method isa method of shifting transmission timings or use frequencies of UEs thatinterfere with each other. A third method is a method of increasing thenumber of repetitive transmissions of the discovery time in order togive interference resistance to the discovery signal (the discoverytime).

In the above embodiment, the example of controlling the discoveryresource quantity by increasing or decreasing the discovery time hasbeen described. However, the adjustment may be performed in thefrequency direction rather than the time direction. Further, thediscovery resources may be adjusted in both the time direction and thefrequency direction. FIG. 16 is a diagram illustrating another format ofthe discovery resources. As illustrated in FIG. 16, the discoveryresources are set to a specific resource block in a specificcommunication frequency band in the frequency direction. Further, thediscovery resources are set to a specific symbol in a specific subframein the time direction.

In the above embodiments, the LTE system has been described as anexample of the cellular communication system, but the cellularcommunication system is not limited to the LTE system, and the presentdisclosure may be applied to a system other than the LTE system.

INDUSTRIAL APPLICABILITY

The present disclosure is useful in mobile communication fields.

1. A method for a mobile communication system that supports adevice-to-device (D2D) communication that is direct inter-terminalcommunication, comprising: transmitting location information indicatinga geographical location of a user terminal located in a cell of a basestation, from the user terminal to the base station; allocating, at thebase station, radio resources to the user terminal based on the locationinformation, wherein the radio resources are used by the user terminalto transmit a signal for the D2D communication; notifying the allocatedradio resources to the user terminal by a unicast manner; receiving, atthe user terminal, a unicast signal indicating the radio resourcesallocated by using the location information, from the base station; andusing the allocated radio resources to transmit the signal for the D2Dcommunication.
 2. A base station for a mobile communication system thatsupports a device-to-device (D2D) communication that is directinter-terminal communication, comprising: a controller configured to:acquire location information indicating geographical location of a userterminal located in a cell of the base station, from the user terminal,allocate radio resources to the user terminal based on the locationinformation, wherein the radio resources are used by the user terminalto transmit a signal for the D2D communication, and notify the allocatedradio resources to the user terminal by a unicast manner.
 3. A userterminal for a mobile communication system that supports adevice-to-device (D2D) communication that is direct inter-terminalcommunication, comprising: a controller configured to: transmit locationinformation indicating a geographical location of the user terminallocated in a cell of a base station, to the base station, receive aunicast signal indicating radio resources allocated by using thelocation information, from the base station, the unicast signal beingspecific to the user terminal, and use the allocated radio resources totransmit a signal for the D2D communication.